The present invention relates to control of interactive video displays provided on a display monitor where these displays can be divided into portions termed "windows" and, more particularly, to such displays in which the contents of "window" portions thereof can be controlled independently for each such "window" provided.
The cathode ray tube, or CRT, as the basis for a video display monitor for television or computers, is well known although other types of display devices such as liquid crystal displays are being increasingly used. Typically, the monitor has a screen for presenting displays, such as the face of a cathode ray tube, over which a raster scanning means is swept, these being focused electron beams for cathode ray tubes. The sweeping activation spot traverses a horizontal line from left to right and then moves down a line to repeat that traversal pattern etc. The resulting sweep pattern is a vertical sequence of lines across the screen, each of which comprises a left to right sequence of picture elements, or pixels. Appropriately switching the electron beams "on" and "off" leads to some of the pixels being irradiated and caused to emit light while others do not to thus form patterns of bright and not so bright pixels, or various colored pixels, across the screen.
Often the desired patterns involves providing textual characters or other symbols overlaying some sort of a background pattern. Such a background pattern may have formed within it pictorial or geometric patterns, and so is often called a graphics background.
One way of displaying textual material over a graphics background is to have the characters of the text directly made part of the graphics background pattern so that they are formed together for presenting displays thereof with any change in one requiring reformulating the whole. The corresponding digital representation of each pixel in such pattern displays can then be stored in a single digital memory arrangement so that these representations have a one-to-one correspondence with the pixels on the screen of the display monitor. A video controller receives the digital representations of the pixels from such a memory arrangement and converts them into suitable signals for controlling the display monitor.
If static scenes having textual characters imbedded therein are to be shown on the screen, such a method is entirely suitable as it is for relatively slow changes in the textual display with respect to the graphics background. However, if the textual characters are to be manipulated rapidly by being changed or repositioned with respect to the graphics background, this method of jointly forming both textual characters and graphics background is slow or expensive, or both, because of the mixing of text and graphics representations in the digital memory arrangement used. For instance, any slight moving of textual characters with respect to a static graphics background requires new digital representations to be stored in the digital memory for every pixel in the display in a manner so as to leave the graphics background the same as it would have been in the absence of text, but with text then embedded in new locations in such a background. The slowness is due to the large amount of storing of new digital representations therein which are required using this method even though the background remains unchanged or changes infrequently, a process which can be accomplished more rapidly only with the use of expensive, faster memories, and possibly with the addition of further expensive auxiliary circuitry.
An alternative way to display textual characters over a graphics background on a display monitor is to store digital representations of the textual character pixels in a digital memory arrangement separate from that in which the digital representations of the graphics background pixels are kept. The digital representations of the textual characters are stored in the text memory in block portions thereof, each such text line block of memory containing a sufficient number of such representations to be coextensive with a horizontal line of text to be displayed on the monitor screen. Each such text line comprises a sequence of column positions to form that line, each of which can contain a textual character or other symbol. In the graphics memory, each screen horizontal pixel line will have a corresponding block of that memory containing a sufficient number of pixel digital representations to be coextensive with that line.
In such a system, a signal from some kind of controller for each horizontal pixel line on the display screen addresses the corresponding blocks in the text memory and in the graphics memory to select those digital representations in each containing the information which is to be used to form the textual character structure portion pixels and graphics background portion pixels for that line. A "character generator" circuit is used to store digital representations of forms of textual characters or other symbols which, in connection with signals from the controller, will provide digital representations of corresponding portions of textual character forms for each screen horizontal pixel line over which the textual character is to extend with the representations for each such portion to be used as digital representations of the corresponding pixels in its screen line. These representations of textual character portions for a screen line are combined with the corresponding background graphics pixels for that screen line in a "mixing" circuit which supplied this combination of digital representations to a video controller to operate the display monitor.
Thus, the textual characters on the screen have a rather limited dynamic relationship with respect to the graphics background. Since only blocks of these two memory arrangements can be addressed by the controller signal, textual characters can change position on the screen in only complete textual line increments up or down but not by screen pixel increments such as a single screen line change up or down.
As a result, there is a desire to provide a display monitor controller which gives a greater dynamic range in manipulating textual characters in respect to the graphics background. In addition, this greater manipulation capability is desired to be combined with a means of variably allocating portions of the screen to independent sources of commands which direct just what graphics background and just what textual characters are to occur on the corresponding allocated screen portion, or "window", on that screen. Further, conveniently changing textual character sizes is also desired.